This invention relates to a manipulator for grasping an object.
1. Field of the Invention
Recently, industrial robots or manipulators are being extensively introduced in various industrial fields for labor saving purpose. In plants, manipulators which can grasp objects and transfer them to a predetermined position are essential.
At present, however, since most manipulators have no judgement function, it is difficult for them to make an action which can be executed very easily by a man. For example, it is difficult for conventional manipulators to grasp deformable objects, e.g., a paper cup.
2. Discussion of Background
Systems which utilize a slippage detector or a weight detector have been proposed for finding an optimum grasping force sufficient to lift up any one of various objects including a heavy and rigid object and a light and fragile object. The former system is disclosed in a paper entitled "Multifunctional Terminal Device with Adaptive Grasping Force", Automatica, Vol. 11, p-p 567-570, Pergamon Press, 1975. In this system, as shown in FIG. 1, a slippage detector 1 is provided on the inner surface of a finger 2 of a grasping mechanism 3. If, when lifting an object 4, the grasping force applied to fingers 2 grasping the object is insufficient, it will begin to slip relative to the fingers. The slippage detector 1 detects the slippage in order to increase the grasping force.
With the system using a slippage detector, if the surface of an object is irregular, there are some cases where it is not possible to detect a slippage. Therefore, it is difficult to perform highly precise grasping force control. If the slippage detector projects from the surface of a finger, the applied grasping force is concentrated on the detector. For this reason, the surface of the detector and that of the finger must be flush with each other. However, this complicates the grasping mechanism. In addition, there is no guarantee that the slippage detector reliably touches a grasped object. Further, due to the presence of the slippage detector the maintenance and inspection of the grasping mechanism become difficult.
The system in which the weight of an object is detected is disclosed in Japanese Patent Publication No. 44744/53 (1978). In this system, since an operation of raising an arm grasping an object, stopping it and then checking whether the object has been lifted, is repeated a number of times, a considerable amount of time is required for determining an optimum grasping force.